3L - oil pressure when hot

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

Hello all

It is very hot here in Georgia, so decided to take my 3L on an extended run to see if it would overheat! The good news is that it did not. However, I did notice that after about 2 hours of running, the oil pressure had dropped to 8psi at idle That did not concern me, but it only increased to about 10 psi when running. Water temperature was 90-95 degrees C.

It has 15-40W oil in it.

Any thoughts?

Thank you

PS - what oil are y'all using?

Chris Card · Guest

Dan - i use 20:50 oil in my 4.5L and the pressure drops from 42 to about 36psi if really hot. My water temp climbs to about 180F in hot weather when moving.
Have you got a bit of dirt in the relief valve seating?

Chris

Paul Spencer · Posted: Thu Jun 16, 2016 10:08 pm Post subject:

An oil thread! This could run and run. Are you on white metal bearings or shells? Do you have a paper oil filter? These affect both the oil you use and the likely pressure. Either way, your oil is thinner than everyone I know is using. It might be worth searching the forum for past discussions.

For comparison, I have shell bearings and a paper oil filter and use Penrite Classic Medium, which is a 25W-70. My oil pressure goes as high as I let it (I have it at 40psi) and never drops below 30psi.

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

Thank you - would you suggest similar pressures in a 3L?

I'll change what's in there now, but can't do too much fiddling just yet as I have another car project which is taking my non-driving time!

Iain Warner · Posted: Fri Jun 17, 2016 12:32 pm Post subject:

It will also depend upon whether you have an early or late engine as the oil pumps are different.
Like Paul I also use 25-70 and have a modern filter in my 3 litre, but am still running with white metal bearings. I have the relief valve set at around 30 psi. If really hot at tickover (~400 rpm) it drops to about 10 psi, but then goes back up to 30 as soon as I start driving again.
Iain

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

This is interesting - the gauge in my 3 liter only goes up to 25.

It is an early engine, not a paper filter, unsure of the bearings.

Today i turned it over on the starter a few times and then started the car. Pressure rose to the usual about 21psi when cold. I did not drive the car or get it warm.

I assume that means there is no blockage of the relief valve (anymore, if there was one).

So, I'm left with these questions:

1.) What is the desired pressure on an early 3L? Gauge is only up to 25.

2.) Should I get 25W-70 (a little difficult but not impossible in the US), or settle for easy to get 20-50? It gets very warm where I live - summers are in the mid 90's routinely.

Many thanks

Robert Craven · Posted: Fri Jun 17, 2016 6:11 pm Post subject:

The Technical Facts at 1.23 says 15-20 with a newly rebuilt engine really hot at 1500 rpm.
That sounds pretty good to me for an unmodified early engine. Mine never went higher than about 15 when really hot, and eventually got lower and lower as the white metal wore to such an extent that I had a new crank, shell bearings and a higher speed pump. The pressure was then about 35-40 (though it's lower now) but that's not a relevant comparison with yours.

PS I use Penrite too (except when I can't get it easily and have to use 20W/50, and Bentley specialist garages have used Castrol GP50 of their own accord). I think Penrite is available in the USA, though you have to be careful to get the right one because they changed the name in the past.

Iain Warner · Posted: Fri Jun 17, 2016 6:52 pm Post subject:

Your oil pressure of 21 psi with the early pump sounds about right as Robert said.
It is only when you get the oil hot that you will be able to judge if there is a bit of muck stuck on the relief valve seat. If the pressure stays at the normal 21 psi then it has probably cleared itself.
Before my engine was rebuilt I ran it on Castrol GP 50, but once rebuilt then switched to a multigrade with a low detergent knowing that the inside of the engine was clean.
Eventually switched to Penrite 25-70 as I was told that the heavier oil provided better protection for the internal gears.
Iain

Paul Spencer · Posted: Fri Jun 17, 2016 7:28 pm Post subject:

Robert mentions getting the right Penrite. It is Classic Medium rather than Shelsey Medium. I have forgotten the difference, but it is more suitable for early engines. The relevance of the paper filter is that I was advised not to use a straight oil with a paper filter. A straight 40 or 50 is fine without, but hard to force through the paper filter when cold. But really, there is no disadvantage with the multigrade apart from the price.

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

I have ordered the Classic Medium - I'll change the oil and inspect the relief valve.

The huge range intuit multi viscosity is a bit of a concern. That required them to add a lot of solids to the original thin oil - possible loss of lubricity.

Thanks everyone

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

I changed the oil to Penrite 25-70, and cleaned out the relief valve. Seems ok now - ran it until hot (ambient temp today over 100) and then set it around 19psi.

The book incidentally says 12 psi at 30mph in top gear.

Robert Craven · Posted: Wed Jul 20, 2016 9:39 pm Post subject:

I too have a copy of a June 1925 Instruction Book which says oil pressure should be 12 psi at 30 mph in top, but that seems a bit low to me, and as I mentioned above the latest Technical Facts says 15 - 20 when really hot at 1500 rpm.
At present I'm using Penrite HPR15 synthetic 15W/60 as an experiment, and on a recent trip to Spain and France in hot weather the pressure was 33 psi or so under normal driving and 35+ at high speed, but that's with shell bearings and a higher speed pump.
If you want to suffer from information overload you could try comparing the various specifications given in the Classic Oils site; eg
Penrite Classic Medium at www.classic-oils.net/Penrite-Classic-Medium-formerly-HPR40
and
HPR15 at www.classic-oils.net/Penrite-HPR15
The viscosity index of the latter is higher.

Robert Clifford-Wing · Posted: Sun Nov 13, 2016 12:57 pm Post subject:

When I purchased our 3 ltr a year ago, I had a full flow external oil filter fitted by the vendors agents, who are restorers of Bentleys.

The oil pressure was set to run at around 50/60 when hot but during the warming up time the pressure will not rise above 40. I assume this is due to the cold oil not being able to be forced through the filter? Reading comments above, is the pressure set too high on on our engine; it is fitted with white metal bearings that appear to be in good condition?

Iain Warner · Posted: Sun Nov 13, 2016 1:15 pm Post subject:

I am no expert but believe that modern full flow filter elements are provided with an internal by pass valve such that if the pressure is set too high (or the filter blocked) the by pass will simply open and you will lose filtration.
To complicate matters filter canisters of similar outward appearance from different manufacturers have different internal relief valve settings.
Suggest that whoever recommended the pressure setting should also be quizzed regarding the compatible filter element to use.
Iain

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

But all that bypass is within the filter - so yes, you lose filtration, but the pressure remains too high in the rest of the system. I've never seen any reference for the 3l or 4.5L that recommends a pressure over 35.

Robert Clifford-Wing · Posted: Mon Nov 14, 2016 2:39 pm Post subject:

Iain Warner · Posted: Mon Nov 14, 2016 6:52 pm Post subject:

Rob
The pressure relief valve setting will determine the pressure at which the sprung loaded ball will lift and divert excess oil directly back into the sump. Below the set pressure, such as when the oil is warm and the engine turning slowly, or if the engine is badly worn, the valve will remain shut and have no effect on oil pressure gauge readings.
However if a bit of grit gets lodged on the relief valve seat stopping the valve from closing it will allow oil to pass without the ball actually lifting then readings below the 'set pressure' will be affected.
I do not know if this explanation has made things clearer. Hope so.
Iain

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

Rob,

The pressure relief valve will affect top end pressure more (i.e. - it will open and allow bypass to the sump when the pressure rises with engine speed) - however: the suggested setting is usually at about 1500 rpm, AND there really should not be a vast difference between idea and running pressure (some certainly, maybe 5-7 pounds)

Chris Card · Guest

Rob and Dan,

There is a minimum pressure reading, which varies between hot and cold, that you will get at idle speeds and an arbitrary maximum which depends on your chosen top end setting at, say, 1500 revs. The difference between the two, will depend on your chosen top end setting - the higher the setting, the greater the pressure range.

On my 4.5L, i"ve set the top at 50psi and the idle pressure varies between about 20psi and 30psi depending on whether the oil (20-50) is hot or cold.

Another factor is the condition and "size" of the oil pump - my 4.5L has a high capacity pump.

Chris

Robert Clifford-Wing · Posted: Wed Nov 16, 2016 7:47 am Post subject:

Paul Spencer · Posted: Wed Nov 16, 2016 9:07 am Post subject:

That's where I set mine on the 3L.

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

My 3 litre is set around 19, as per the book.

Chris: on my 4.5 litre, i adjusted the idea down today after the car was nice and warm, to around 600 (it will idle easily at 450 and I may go there after a few test runs) - but I noticed my idle pressure dropped to around 15. Top end is around 35. Oil is 25-70. I'm a little worried at that low end pressure.

John Murch · Posted: Fri Nov 18, 2016 6:39 am Post subject:

When hot my car idles with about 15psi, and at circa 1200 its 37psi. This is with a high capacity pump and new shells.
In the distant past I had a 4.5 which had done a huge milage, at idle, pressure was barely discernible and at speed circa 20, I ran it for 25,000 miles in this condition. John Guppy (McKenzie Guppy) assured me that it was fine and oil flow was more important than pressure. Rapid changes in pressure being the thing to look out for.
John

Chris Card · Guest

Dan, I don't think you need to worry - are you on white metal bearings, or shells? I don't know whether, or not, that makes any difference.

One thing that can give you spurious readings is air in the pipe to the pressure gauge. If you suspect that, disconnect at the gauge with the engine idling and any air will come out with some oil, then reconnect. It is a bit messy, but if you're prepared with some rags etc it's OK.

Chris

Dan Suskin · Joined: 22 Feb 2011 Posts: 298 Location: Georgia, USA

Thanks John and Chris.

Iain Warner · Posted: Fri Nov 18, 2016 9:54 am Post subject:

Dan,
I think your idle oil pressure is perfectly respectable, in your initial post it was only 8psi and that was at the higher revs. I guess the difference is due to your change of oil spec and presumably there was a bit of muck in the relief valve that you removed.
Iain

Iain Warner · Posted: Fri Nov 18, 2016 9:57 am Post subject:

I have just spotted that we are talking about two different cars ( 3L and 4.5L). Apologies for the confusion.
Iain